With the passing of zero emission vehicle legislation, electric vehicles are emerging as an important market for induction motor drives. A rather special requirement of electric vehicle drives is the desirability of operating the drive continuously within a constant horsepower over a very wide speed range corresponding to that of "steady driving" or "cruising". Hence, the issue of good efficiency during field weakened operation becomes one of paramount importance for electric vehicles. FIG. 1 shows a typical family of torque speed curves for variable frequency operation of an induction machine assuming that the inverter voltage becomes a constant amplitude above one per unit speed and that a four to one field weakening range is required. It is clear from the FIG. 1 that, since the torque varies as the square of the voltage, to reach a torque of 0.25 per unit at four time rated speed, the machine must be capable of 4 per unit torque at rated speed. Since machines of a typical design have only a 1.5 to 2.5 per unit breakdown torque, such a machine must be oversized by roughly a factor of two simply to enable it to reach four per unit speed in the field weakening range at rated power. Thus, it becomes apparent that machines designed for traction type applications become bulky as the constant horsepower speed range increases.
In addition to simply over-sizing the machine, a wider speed range has previously been accomplished by various pole changing techniques involving contactor switching. The machine can be wound with two stator windings having a different pole number. When one winding is in use, the other is open circuited and vice versa.
Another scheme is the pole change winding, where a single winding is reconnected to obtain a two-to-one pole ratio. In addition to reversing certain coil groups, the reconnection might include changing the coil groups from series to parallel and the connections among the phases from Y to .increment. or vice versa depending on the desired torque-speed characteristics.
Another previously suggested technique is pole amplitude modulation where pole numbers differing by ratios other than two are obtained by switching unsymmetrically distributed windings. In this case, the poor air gap MMF distribution makes this technique impractical except when efficiency is of no concern.
The speed range of an induction motor drive can also be extended by winding tap changing with contactors. For lower speed range all the winding turns are used while for operation at high speeds the tapped turns only are used.
The present invention is the further input in the art eliminating deficiencies of the prior art and allowing contactless pole changing technique to extend a speed range.